In addition to an optical disk having a single recording layer as in a CD, a multilayer optical disk having a plurality of recording layers for increasing recording capacity as in a DVD and a high-density DVD has come to be used. However, when information is read (hereinafter called “reproduced”) from a recording layer of the multilayer optical disk, stray light reflected from a recording layer not to be subjected to reproduction (a non-reproduction layer) as well as information light reflected from a recording layer to be subjected to reproduction (a reproduction layer) enter a photodetector. Therefore, interference between the information light and the stray light sometimes leads to deterioration of the accuracy of reproduction.
FIGS. 8A and 8B are ray diagrams achieved when a DVD 808 having two recording layers is subjected to reproduction. FIG. 8A shows a light path of light reflected when a first recording layer 809 close to a light incidence plane is subjected to reproduction, and FIG. 8B shows a light path of light reflected when a second recording layer 810 distant from the light incidence plane is subjected to reproduction.
Specifically, the light reflected by the DVD 808 passes through an objective lens 804 and a collimator lens 803, to thus enter a photodetector 805.
As shown in FIG. 8A, light-source light emitted from the light source comes into a focus on the first recording layer 809 at the time of reproduction of data in the first recording layer 809, and the light-source light reflected from the first recording layer 809 turns into information light 801 and converges on a photodetector 805. However, the light-source light does not come into a focus on a second recording layer 810, the light-source light reflected from the second recording layer 810 turns into stray light 802 and converges on a position ahead of the photodetector 805 and does not converge on the photodetector 805; however, a portion of the stray light enters the photodetector 805.
Further, when the second recording layer 810 is subjected to reproduction as shown in FIG. 8B, the light-source light comes into a focus on the second recording layer 810, and hence the information light 801 reflected from the second recording layer 810 converges on the photodetector 805. However, the light-source light does not come into a focus on the first recording layer 809, the stray light 802 reflected from the first recording layer 809 converges on a location behind the photodetector 805. Hence, the stray light does not converge on the photodetector 805, but a portion of the stray light enters the photodetector 805.
Consequently, the information light reflected from the reproduction layer and the stray light reflected from the non-reproduction layer interfere with each other within the photodetector 805; hence, deterioration of the accuracy of reproduction is unavoidable.
In order to enhance the accuracy of reproduction, there has already been proposed an optical member used for diminishing the interference between the information light reflected from the reproduction layer and the stray light reflected from the non-reproduction layer (see; for instance, JP-A-2005-203090).
As will be shown in FIG. 9A, an optical member 9 of the proposal has a hologram area 91 that is formed in a center portion of an effective region 901 and is essentially identical with the photodetector 805 in terms of an outer shape.
The light-source light is reflected by the reproduction layer, to thus turn into information light; and is reflected by the non-reproduction layer, to thus turn into stray light. The information light and the stray light that enter the hologram area 91 of the optical member 9 undergo diffraction in the hologram area 91 and do not enter the photodetector 805.
Consequently, as shown in FIG. 9B, the area of the photodetector 805 is not exposed to the stray light 802 within the plane of the photodetector 805, and the stray light diffuses into a circumference 94 of the photodetector 805.
In the meantime, the information light 801 entered the hologram area 91 undergoes diffraction in the hologram area 91, to thus fail to reach the photodetector 805. However, the information light 801 transmitted through an area other than the hologram area 91 creates spots 93 on three cells 92 of the photodetector 805. Therefore, the information light 801 and the stray light 802 does not interfere with each other within the photodetector 805, and the accuracy of reproduction of the multilayer optical disk can be enhanced.
However, in the optical member of the proposal, the hologram area 91 diffracts the information light 801 and the stray light 802 regardless of a wavelength. When an optical disk (e.g., a CD) having a single recording layer which does not require prevention of interference is subjected to reproduction, information light that has a high density of light and that is acquired in the vicinity of an optical axis is lost, which raises a problem of a decrease in a utilization ratio of light. An optical element may also be retracted from an optical path at the time of reproduction of an optical disk during which prevention of interference is not required. However, there must be provided a mechanism that inserts and retracts an optical element according to the type of an optical disk to be subjected to reproduction.